Mechanisms for converting rotary motion to reciprocating linear motion and vice versa have been known heretofore. For example, W. E. Rise U.S. Pat. No. 3,217,901, dated Nov. 16, 1965, shows a transfer mechanism that uses a barrel cam having a curved slot therein and a cam follower of the roller type that runs in such slot for converting rotary motion to linear motion. Also, J. Dietlin U.S. Pat. No. 3,264,949, dated Aug. 9, 1966, shows an apparatus for transforming a rectilinear movement into a rotary movement that uses a hollow cylindrical section provided with a helical slot into which a pin extends so that when the pin is moved, this pin will act on the walls of the slot formed in the cylindrical section so that it is constrained to revolve without it being simultaneously constrained to move axially. While these power devices have been useful for their intended purposes, they have nevertheless been handicapped by certain disadvantages such as high force gradient with angular displacement, being rather complicated in design, and subject to considerable wear in use. Accordingly, it has been found desirable to provide improved means for converting rotary motion to reciprocating linear motion that does not have these disadvantages as hereinafter described.